Isotopes of xenon

Naturally occurring xenon (54Xe) consists of seven stableisotopes and two very long-lived isotopes. Double electron capture has been observed in 124Xe (half-life 1.8 ± 0.5(stat) ± 0.1(sys) ×1022 years)[1] and double beta decay in 136Xe (half-life 2.165 ± 0.016(stat) ± 0.059(sys) ×1021 years),[2] which are among the longest measured half-lives of all nuclides. The isotopes 126Xe and 134Xe are also predicted to undergo double beta decay,[4] but this has never been observed in these isotopes, so they are considered to be stable.[5][6] Xenon has the second-highest number of stable isotopes. Only tin, with 10 stable isotopes, has more.[7] Beyond these stable forms, 32 artificial unstable isotopes and various isomers have been studied, the longest-lived of which is 127Xe with a half-life of 36.345 days. All other isotopes have half-lives less than 12 days, most less than 20 hours. The shortest-lived isotope, 108Xe,[8] has a half-life of 58 μs, and is the heaviest known nuclide with equal numbers of protons and neutrons. Of known isomers, the longest-lived is 131mXe with a half-life of 11.934 days. 129Xe is produced by beta decay of 129I (half-life: 16 million years); 131mXe, 133Xe, 133mXe, and 135Xe are some of the fission products of both 235U and 239Pu, so are used as indicators of nuclear explosions.

Relatively high concentrations of radioactive xenon isotopes are also found emanating from nuclear reactors due to the release of this fission gas from cracked fuel rods or fissioning of uranium in cooling water.[citation needed] The concentrations of these isotopes are still usually low compared to the naturally occurring radioactive noble gas222Rn.

Xenon-124 is an isotope of xenon that undergoes double electron capture to tellurium-124 with a very long half life of 1.8×1022 years, more than 12 orders of magnitude longer than the age of the universe ((13.799±0.021)×109 years). Such decays have been observed in the XENON1T detector in 2019, and are the rarest processes ever directly observed.[10] (Even slower decays of other nuclei have been measured, but by detecting decay products that have accumulated over billions of years rather than observing them directly.[11])

Xenon-136 is an isotope of xenon that undergoes double beta decay to barium-136 with a very long half life of 2.11×1021 years, more than 10 orders of magnitude longer than the age of the universe ((13.799±0.021)×109 years).